Trump will bring it to the SC. If he loses big he will attack as he has the legality of mail in voting and have his SC say they are illegal and therefore Trump wins. If Trump loses by a small margin in a few states he will ask for recounts or if there is to big a lead for a recount, but it is close, he will ask republican controlled legislatures to put in their own electors to vote for him instead of Biden. Then he will go the SC who will say he was allowed to do that, and that it is valid because something about the true will of the people.
If Biden wins and Trumps shenanigans don't somehow work, and the SC doesn't go along with him. Trump will be forced out and as
@crandc said he will claim to be president in exile or the true president.
He will go to Florida his "home state" and use Mar a Lago as his whitehouse. He will be surrounded by his loyal minions. The proud boys and other right wing groups will be his secret service.
Several states loyal to Trump will secede and he will become the 2nd President of the Confederate States of America.